In the rainforest, the diversity of insect species is often significantly greater than that of all other animal groups combined, a phenomenon that perplexes many entomologists.
Correct: C
The original sentence (Option A) makes a logically sound comparison: 'the diversity of insect species' is compared to 'that of all other animal groups combined,' where 'that' correctly stands for 'diversity.' This is a grammatically correct and common comparative construction. However, Option C offers a slightly more direct and concise way to express the same comparison: 'significantly greater than the combined diversity of all other animal groups.' By directly stating 'the combined diversity,' it avoids the slight redundancy of 'that of' when the full noun phrase is already present and clearly parallel. Both are correct, but GMAT often favors conciseness and directness when meaning is preserved. Option B and D create an illogical comparison between 'diversity' and 'animal groups.' Option E uses 'more significant,' which is less precise than 'significantly greater' when referring to magnitude, and incorrectly uses 'which' for a restrictive clause ('a phenomenon that perplexes'). Therefore, Option C, while very close to A, is marginally more direct and concise.